Electric resistance element



[UNITED STATES- PATENT OFFICE.

LEON MCCULLOCH, OF WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTING- HOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION 01' Patented Jan. 11, 1921.

PENNSYLVANIA.

ELECTRIC RESISTANCE ELEMENT.

1,365, Specification Letters Patent.

No Drawing.

To all whom it may concern Be it known that I, LEo MCCULLooH, a citizen of the United States, and a resident of Wilkinsburg, in the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in Electric Resistance Elements, of which the following is a specification.

My invention relates to resistance elements for 'use in electric apparatus, such as rheostats, heating devices and the like, and it has for its general object to provide resistance elements of great durability under service conditions.

More particularly, my invention aims to utilize retsistance conductors which have not heretofore been practically available for this use on account of their rapid deterioration when subjected to electric current under operating conditions.

.Many conductors of electricity are suitable for use as resistance elements, so far as their electrical characteristics are concerned, but relatively few of such conductors have heretofore been of practical utility because of the degree to which most of them become oxidized at elevated temperatures. Numerous attempts have been made to coat resistors with enamels, glazes or cements, but this expedient has been satisfactory only in the case of resistors intended for operation at relatively low temperatures, on account of the fact that the protective coatings must be" applied in liquid form or must be liquefied during the coating process,- and,

when the coated resistor is heated, it has been found that the coating will again liquefy and drop from the conductor.

According to my present invention, I produce satisfactory coated resistors by utilizing a class of coating materials which I have ploy makes it possible, not only to apply' permanent coatings to conductors which are found to possess the property of softening under moderate heat, but melting to complete fluidity only at temperatures above the temperatures present in the ordinary forms of electric resistanceapparatus. This peculiar property of the coatings whichI emafterward to be highly heated, but to provide a coating which is flexible and not likely to crack "under variations of temperature, as most coating materials will do.

Application filed June 8, 1916. Serial No. 102,481.

The coating substances which I prefer to employ are compounds of boron, particularly its oxygen compounds such as the borates, metaborates and oxids. Many conducting materials are suitable foruse in connection with these protective coatin s, such, for example, as nickel steel and 0t er iron alloys, as well as non-ferrous alloys and all other conducting substances having high a of ammonium orthoborate, preferably a saturated, or at least a concentrated, solution and preferably with the addition of an excess of ammonium .hydroxid. This solution may be at once applied to a portion or all of the surface of the resistor to be coated, either by dipping or by brushing, or it may be mixed with a small amount of any suitable filler such as kaolin, metallic oxids, or clays of the bentonite type. The coating mixture may be applied to the resistance material before the material is given its final form, but I find it convenient to first shape the resistor or mount it upon an insulating support and to apply the coating as a final step. For example, resistance elements for heating apparatus such as sad irons and percolators are commonly formed by wind ing a wire or ribbon of a resistance con ductor upon an insulating support of :mica, lava or the like, and, in such cases, the'coating material may be applied to the entire.

structure.

After the preliminaryfcoating described above is applied to the resistor, the latter is subjected to heat to decompose the ammonium borate and to drive off the ammonia and water from the coating.- This heat treatment may be suitably carried out at'a temperature of about 500 C. and be continued until substantially all the ammonia and water are expelled, leaving a coating of boron trioxid which is inert to the metal. The resistor may then be used without further treatment, or an additional coating of refractory cement may be applied over the boric oxid coating as a mechanical protection and also to prevent free access of moisture to the boric oxid.

A suitable solution for use in carrying out the above process may be made by dissolving about 65 grams of an ammonium borate in 100 grams of hot water and adding about 20'cubic centimeters of concentrated ammonium hydroxid. A smaller proportion of ammonium borate may be employed, but I find that better results are obtained if the solution is concentrated or even saturated.

The boric oxid coating may be obtained in a number of ways in addition to the method described above. For example, the boric oxid, in powdered form, may be sprinkled upon the resistor to be coated, which is then heated to soften the boric oxid sufficiently to permit it to be spread in an even layer over the surface of the resistor. As stated above, the boric oxid will soften and flow One sultable paste of this 'class is made by triturating about 700 grams of ammonium orthoborate in a mixture of about 950 cubic centimeters of water and about 150 cubic centimeters of ammonium hydroxid, sp. gr.

' 0.92. These proportions may be varied within wide limits. The flowing properties of these pastes may be increased by the addition of a small amount, two or three per cent, of a clay of the bentonite type.

In addition to boron trioxid, other boron compounds are suitable for use according to my present invention, for example, the metallic borates, such as lead borate, sodium borate, sodium metaborate and borax.

Some of the boron compounds which -I have mentioned possess the advantage, in addition to their behavior under the action of heat, that they have but little fluxing effect upon other insulating materials such as mica, and may therefore be used in contact with them. Furthermore, these coatings, while belonging to the class of materials in which electric resistivity decreases with increase in temperature, exhibit this property but little within the range of temperatures encountered in ordinary resistance apparatus.

One advantage of the use of some low melting compound to surround the heating element embedded between mica plates is the fact that the heat conductivity of the insulation is considerably increased, causing the resistor to operate at a lower temperature for the same heat output. All of these properties combine to increase the efficiency of resistors provided with these coatings. In view of the considerable range of materials and process steps which may be employed without exceeding the limits of my invention, it .is 'to be lunlerstood that the scope of my invention is limited only by that of the appended claims.

1 claim as my invention:

1. An electric resistance elenicut comprising a conductor of electricity having at least a portion of its surface covered with a coat ing material that softens, but does not fully melt, at the operating temperature of the said resistance element.

2. An electric resistance clement comprising a conductor of electricity having at least a portion of its surface covered with a coating material that softens at a temperature below 500 C. but melts at a temperature above 500 C.

3. An electric resistance element comprising a conductor of electricity having at least a portion of its surface covered with a coating consisting essentially of a compound of boron.

4. An electric resistance clement com rising a conductor of electricity having at east a portion of its surface covered with a coating consisting essentially of an oxygen compound of boron.

5. An electric resistance element comprising a conductor of electricity having at least' a portion of its surface covered with a coating com rising a com and of boron that softens, utdoes not ully melt, at a temperature below the operating temperature of 1 the said resistance element.

6. An electric resistance element comprising a conductor of electricity having at least a portion of its surface covered with a coating consisting essentially of boron trioxid.

7. An electric resistance element comprising a conductor of electricity having at least a portion of its surface covered with a coating comprising a compound of boron, the said coatlng being covered with cementitious material.

8. An electric resistance element comprising a conductor of electricity having at least a portion of its surface covered with a coating comprising a compound of boron that softens, but does not fully melt at a temperature below the operating temperature of the said resistance element, the said coating being covered with cementitiousmaterial.

9. An electric reslstance element comprising a conductor of electricity having at least a portion of its surface-covered with a coatmg comprising an oxygen compound of boron, the said coating belng covered with 'cementitious material. I

10. An electric resistance element comprising a conductor of electricity having at least a portion of its surface covered with a coating comprlsing boron trioxid, the said coating being covered with cementitious ma- I prising a body of nickel steel having at least a portion of its surface covered with a coating comprising an oxygen compound of boron. v

14. An electric resistance element comprising a body of nickel steel having at least prising a body 'gether with substantially 2 to 3% a portion of its surface covered with a coating comprising boron trioxid.

15. An electric resistance element comprising a conductor of electricity having at least a portion of it surface covered with a coating comprising an oxygen compound of boron and a small proportion of bentonite.

16. An electric-resistance element comof nickel steel with an oxidization-preventing :coating of a compound of boron.

17. An electric-resistance element comprising a conductor of electricity, having at least a portion of its surface covered with a coating comprising a compound of boron and a small amount of a filler. v 18. An electric-resistance element comprising a conductor of electricity, having at least a portion of its surface covered with a coating of a compound of boron together with a small amount of a clay as a filler.

19. An I electric-resistance element 'comprising a conductor of electricity, having at least a portion of its surface covered with a coating comprising a compound of boron toof bentonite. y

In testimony whereof I have hereunto sub scribed my name this 26th day of May, 1916.

LEON MOGULLooi-I; 

